I find an easy way to make the geek/nerd line definable is that when a geek does something he tends inspire and wow other people in what ever topic it tends to be. A nerd is someone who could do the same thing but tends to creep people out or offend them.

Example: Big bang theory. Leonard is a geek. Sheldon is a nerd.

You can play D & D and be a geek about it and try to get others to play and want everyone to have fun or be a nerd about it and dress up and get so much into character that everyone just thinks your creepy.

I find an easy way to make the geek/nerd line definable is that when a geek does something he tends inspire and wow other people in what ever topic it tends to be. A nerd is someone who could do the same thing but tends to creep people out or offend them.